Annex D of the “ATSC Digital Television Standard” was published by the Advanced Television Systems Committee (ATSC) in September 1995 as its document A/53. This standard defined the broadcasting of digital television (DTV) signals within the United States of America and is referred to in this specification simply as “A/53”. A/53 specifies a vestigial-sideband amplitude-modulation signal in which the digital symbols are transmitted by eight-level modulation known as 8VSB, which has −7, −5, −3, −1, +1, +3, +5 and +7 normalized modulation signal values. The digital symbols are subjected to 2/3 trellis coding. The transmission of more robust DTV signals at one-half or one-quarter the code rate of ordinary 8VSB signals subsequently became a subject of interest at the beginning of the twenty-first century.
Such transmissions entailed sacrifices of so much channel capacity that apparently most broadcasters have subsequently judged them not to be viable from a business standpoint. Furthermore, proposed systems for transmitting more robust DTV signals at one-half or one-quarter the code rate of ordinary 8VSB signals have attempted to limit these transmissions to windows of 184-byte duration in the 207-byte segments of data fields. This limitation leads to the transport stream multiplexer in a DTV transmitter becoming so complex as to be impractical.
Previously, as documented in U.S. patent application Ser. No. 11/724,364 filed 15 Mar. 2007 and titled “ROBUST DTV SIGNALS THAT CAN OVERCOME BURST ERRORS UP TO 1040 BYTES OR MORE IN LENGTH”, as well as in the similarly titled provisional U.S. patent application Ser. No. 60/782,481 filed 15 Mar. 2006, the inventor solved the problem concerning the transport stream multiplexer being too complex to be practical. The 207 bytes of a (207, 187) Reed-Solomon codeword to be transmitted at the code rate of ordinary 8VSB signal occupy just a single segment of a data field. The RS codeword is convolutionally interleaved to separate its bytes so that they occur at 52-byte intervals in the convolutional interleaved signal supplied for trellis encoding that reduces code rate by a factor of 2/3. This spreads the RS codeword over fifty-two 207-byte segments of a convolutionally interleaved data field. The inventor discerned that a (207, 187) Reed-Solomon codeword to be transmitted at a code rate one-half that of ordinary 8VSB signal would occupy exactly two 207-byte segments of a data field before convolutional interleaving. By including the initial half of the codeword fifty-two 207-byte segments of a data field earlier than its final half, subsequent convolutional interleaving spreads the RS codeword over one hundred four 207-byte segments of the convolutionally interleaved data field. The inventor further discerned that a (207, 187) Reed-Solomon codeword to be transmitted at a code rate one-quarter that of ordinary 8VSB signal would occupy exactly four 207-byte segments of a data field before convolutional interleaving. By beginning the four segments at 52-data-segment intervals, subsequent convolutional interleaving spreads the RS codeword over two hundred eight 207-byte segments of the convolutionally interleaved data field. Accordingly, the operation of the transport stream multiplexer in the transmitter only needs to take into account a duration somewhat shorter than a single data field time, rather than the duration of many data fields. So, the data memory required to support transport stream multiplexing can be reduced to practical size.
In U.S. patent application Ser. No. 11/119,662 filed 2 May 2005 and titled “DIGITAL TELEVISION SIGNALS USING LINEAR BLOCK CODING” the inventor described the use of linear block codes for halving the code rate of DTV signals. In U.S. patent application Ser. No. 11/724,364 the inventor described the separation of the parity bits of systematic linear block codes from the information bits, so that (207, 187) Reed-Solomon codewords containing just the information bits could be usefully received by legacy DTV receivers. Patent application Ser. No. 11/724,364 indicated the systematic linear block codes were preferably an (8, 4) extended Hamming code or a (16, 8) code derived therefrom. These linear block codes can locate erroneous bytes for the (207, 187) Reed-Solomon forward-error-correction coding, which permits a decoding algorithm to be used that can correct up to twenty erroneous bytes. D. A. Luthi disclosed this decoding algorithm in U.S. Pat. No. 5,875,199 issued 23 Feb. 1999 and titled “Video Device with Reed-Solomon Erasure Decoder and Method Thereof”.
Apparently, transmitting robust signals within just a 184-byte window in each 207-byte data segment was done to accommodate legacy receivers identifying the PIDs of the packets containing robust signals as PIDs not associated with ordinary 8VSB transmissions. So the legacy receivers can do this accurately, the packets are Reed-Solomon-coded. This approach presents an 11.1% cost in overhead, before the information code rate is reduced.
It is not necessary to use PIDs to make legacy DTV receivers disregard a data segment. If one wants legacy DTV receivers to disregard a data segment, all that is necessary is that the data segment not be a correct or correctable (207, 187) Reed-Solomon codeword. The data segment will then be discarded in the transport stream de-multiplexing process of a legacy DTV receiver. Data are subject to randomization by exclusive-ORing them with a prescribed pseudo-random binary sequence. So, if a segment of robust data undesirably appears to be a correct or correctable (207, 187) RS codeword for ordinary 8VSB, often simply sending the data during different data-segment intervals can cure the problem. However, this complicates the transport stream multiplexer.
Rather than moving the data to a different data-segment interval, twenty or so selected bytes of each 207-byte chunk can be modified in a prescribed way before transmission, so the chunk will not be mistaken for be a correct or correctable (207, 187) RS codeword. The 207-byte chunk can then be restored after reception. The overhead required to send information concerning which segments are modified is less than a percent. Furthermore, trial-and-error RS decoding will allow the 207-byte chunks to be restored after reception without having to receive specific other information about the modifications.
High-definition-television (HDTV) signals can be satisfactorily transmitted using somewhat less than two-thirds the capacity of the DTV channel. The (15, 11) Hamming code can be shortened to a (12, 8) cyclical linear code, capable of arrangement as a systematic code in which the original information bits appear in their original order. The (12, 8) cyclical linear code can be used to reduce code rate to two-thirds the code rate of ordinary 8VSB signal. The inventor discerned that this permits a more robust transmission of the entire HDTV signal, which should be a viable commercial use of coding of a DTV signal to increase the redundancy therein. Furthermore, the inventor discerned, the parity bits for the linear coding of each successive non-overlapping pair of (207, 187) RS codewords could be transmitted in another 207-byte segment of data. This would permit legacy DTV receivers to receive the entire HDTV signal usefully, though not as robustly as a DTV receiver especially designed to utilize the redundant linear block coding of the HDTV signal. Alternatively, the same coding scheme could be applied to two or possibly three standard-definition-television (SDTV) signals. Still further, the same coding scheme could be applied just to selected portions of the television signal(s), such as the audio portion.